9 Ceramic and Other Hard Coatings

“Hard and other ceramic coatings” is a very broad topic, with entire books written on the subject [1,2]. In addition, several recently published books on tribology have included chapters on hard coatings [3, 4, 5]. This does, however, underscore the importance of and interest in the subject. The reason for the attention on hard coatings goes back to the fundamentals of tribology. Materials in intimate contact and in relative motion dissipate energy and wear. Chemical reactions at these interfaces are accelerated, often beyond what's expected from “flash temperatures” due to frictional heating. It is thought that the distortion of chemical bonds, creation of defects and generation of high surface-to-volume particles, among other factors, assist in driving chemical reactions. Clearly, friction and wear cause the consumption of resources and both processes occur at surfaces. The economic incentive for controlling friction and wear has led to growth of the tribological sciences and, for the purpose of this chapter, to the growth of surface engineering, where hard coatings are included.

There are many different types of coating processes spanning electrochemical, sol-gel, thermal and plasma spray, physical vapor deposition and chemical vapor deposition. Coating techniques for producing thin and thick coatings is a broad topic in itself and is beyond the scope of this chapter. The different processes have developed because each addresses a specific need. In general, processes are suited for providing either very thin (µm-level) or very thick (mm-level) coatings. Thin-film coating deposition has undergone tremendous advances and it is possible to grow multilayered, functionally gradient and nanocomposite coatings that have impressive properties. Nanostructured coatings in particular produce superhardness and excellent toughness; other designs permit combination of hardness, toughness and low friction across multiple environments (e.g., low/high temperature and moist/vacuum). While thin film coatings have many intriguing properties, thicker coatings still hold much interest because they enable the repair and rebuilding of worn parts. However, this chapter will be limited to thin-film coatings.

One of the primary functions of a hard coating is to protect surfaces from wear. Hardness at a very basic level is described by the ability of one material to scratch another. Considering this fact, it is not unexpected that some of the first uses of hard coatings (e.g., TiN) were to extend the life of cutting tools. A review of wear mechanisms in the next section clearly shows that hardness is a desirable property. However, hard coatings can cause high wear on mating surfaces. To control system wear, careful selection of lubrication schemes and counterface materials is necessary, otherwise, the wear process simply transfers to the contacting material. In addition, hard coatings may be brittle, which can limit applied load and require the coatings to be very thin (e.g., less than 1 µm). More recently, coatings that are hard, but also exhibit high toughness and low friction, have been developed. These coatings can reduce the severity of wear on the counterface and reduce the detrimental effects of high shear stress in the contact.